Device for a hydraulic cutting tool

ABSTRACT

A device for a hydraulic cutting tool ( 30 ) for cutting of at least one pipe ( 16, 18, 20 ) or tubular object beneath a water floor ( 10 ), wherein the cutting is carried out from a surface facility ( 2 ). The cutting tool ( 30 ) is anchored in a pressure tight manner in the pipe ( 16 ), whereupon pressurised gas ( 86 ) is pumped into a pipe section ( 80 ) immediately below the cutting tool ( 30 ). With this, liquid ( 8 ), and eventually pressurised gas ( 86 ), will flow out of the pipe section ( 80 ) via a short drain pipe ( 66 ) through the cutting tool ( 30 ), so as to create a gas filled pipe volume ( 90 ) comprising the relevant cutting depth ( 32 ) in the pipe section ( 80 ). The invention differs from prior art by the outflowing liquid being led onwards up to the surface facility ( 2 ) via a drain line ( 92 ). The upper end portion of the drain line ( 92 ) is connected to at least one adjustable choke device ( 102, 104 ). This allows the gas overpressure in said pipe volume ( 90 ) to be controlled during the cutting, providing optimal operating conditions during the hydraulic cutting.

BACKGROUND OF INVENTION

1. Field of Invention

This invention regards a device for a hydraulic cutting tool for cuttingof tubular objects beneath a water floor, e.g. beneath a seafloor.

The device is preferably used for cutting of casings in connection withthe permanent plugging and abandonment of a well drilled under water,e.g. a petroleum well. After cutting, the cut-off pieces of casing maybe removed from the water floor. Such a well may be completed at thewater floor or above water, e.g. on a platform or another type ofsurface facility. In the latter case the well is connected to thesurface facility via a riser. On the other hand, both types of wells aredrilled under water and down into a water floor, and such a well ishereinafter termed an offshore well.

Said device may also be used in connection with the cutting of othertypes of tubular objects disposed in a water floor. Such an object maycomprise a tubular pile or a caisson. As an example, tubular piles areused to anchor platforms and other offshore structures to a water floor.In this case, the piles are driven into the water floor, then to befixed to appropriate fixing devices such as fixing brackets on theoffshore structure in question.

2. Description of the Related Art

The invention is based on the cutting of casings, in particular compoundcasings, beneath a water floor upon abandonment of offshore wells. Whencutting beneath a water floor, access from the outside of the pipes isimpossible, making it necessary to perform the cutting from inside thecasing. In this connection, known cutting devices and cutting methodsare encumbered with a number of disadvantages and problems.

Cutting of said tubular objects under a water floor, including casings,piles and caissons, is normally carried out mechanically, hydraulicallyor through blasting.

As the invention comprises a hydraulic cutting tool that is known perse, and which is typically used for cutting of casings in an offshorewell, the following discussion will only concern hydraulic cutting ofcasings according to prior art. This discussion also concerns thosedisadvantages of known hydraulic cutting techniques which the presentinvention seeks to remedy. This is also necessary in order to understandsignificant characteristics of the invention, as well as the problemswhich the invention seeks to remedy.

A well is normally composed of several casing strings arranged insideeach other with decreasing diameters, where each smaller casing stringextends deeper into the ground than the previous and larger casingstring. In addition, one or more annuli between the casing strings maybe completely or partially filled with set cement. Such casing stringsare hereinafter only termed casings.

The cutting of casings beneath a water floor is carried out by means ofa hydraulic cutting tool which is lowered into the well from a surfacefacility such as a platform, the cutting tool being lowered to therelevant cutting position in the innermost casing of the well. Thecutting tool is equipped with a high pressure nozzle through which aconcentrated jet of fluid exits at high speed, cutting through thecasing and any annular cement. The exiting high speed jet normally has adiameter of 1–2 mm and is delivered at a very high pressure, for example1000 bar. The cutting jet consists of a fluid, preferably water, mixedwith an abrasive. The cutting fluid is hereinafter termed an abrasivefluid. According to prior art, such hydraulic cutting is carried out atwater depths of up to 100 meters, and the cutting is often carried out 5meters beneath the water floor. Moreover, the cutting method isrelatively quick, requires little equipment, and may be carried out witha minimum risk of injury/damage to personnel, servicing means and anyremaining downhole equipment, including well plugs that seal against anyreservoir fluids.

In principle, a conventional hydraulic cutting system consists of a highpressure pump; a mixing device in which said fluid and abrasives aremixed; a cutting tool comprising among other things said high pressurenozzle, a high pressure line through which said abrasive fluid is pumpeddown to the cutting tool; at least one auxiliary line via which e.g.hydraulic and/or electrical driving power and/or hydraulic/electricalcontrol and/or monitoring signals are transmitted to the cutting tool;and a hoisting device such as a wire winch for bringing the cutting toolinto or out of the well. In addition, the cutting tool comprises anactuator, preferably hydraulically actuated, for fixing and possiblysealing the cutting tool in the casing in question; a rotating motor,preferably hydraulically actuated, for rotating the high pressure nozzleduring the cutting; and various other known equipment such as sprockets,shafts, bearings, gears, clamping implements, gaskets, hydrauliccylinders and pistons, pipes, couplings, control units and monitoringequipment. Operation of said rotating motor and actuator depends amongother things on there being auxiliary lines available through which saiddriving power and control and/or monitoring signals may be transmittedto the cutting tool.

From the surface facility and in the innermost casing of the well, thecutting tool, said high pressure line for abrasive fluid and saidauxiliary lines are lowered to the cutting position beneath the waterfloor. Then the cutting tool is fixed against the wall of the casing inthe working position by at least one associated hydraulically actuatedand releasable anchoring device, e.g. a clamping jaw or a clamping claw.The cutting tool may also be equipped with at least one hydraulicallyactuated and releasable anchoring-and sealing device, e.g. at least onerubber elastic packing, which is pressed against the casing wall andseparates two sections of the casing in a pressure tight manner. In thelatter case, the anchoring device and the sealing device may be actuatedby a common hydraulic actuator device driven and controlled by means ofsaid auxiliary lines.

Hydraulic cutting is initiated by the abrasive fluid being pumped fromsaid high pressure pump and down through said high pressure line to thecutting tool. The abrasive fluid is conducted further through thecutting tool to an angular and rotatable high pressure pipe, the freeend of which is connected to said high pressure nozzle, the highpressure pipe and the nozzle projecting down from the cutting tool. Bymeans of a rotating motor and suitable transmission means, said pipe andnozzle are rotated peripherally through at least one complete rotation(at least 360° angle) about the longitudinal axis of the casing. Thehigh pressure pipe and the nozzle are rotated at an appropriateperipheral speed, and preferably in the horizontal plane, the cuttingjet simultaneously cutting through one or more casings and any annularcement. In this connection at least one annulus may be completely orpartially filled with set cement, liquid and/or air.

According to prior art, the hydraulic cutting is generally carried outin an environment consisting of the liquid normally present in theinnermost casing, e.g. seawater. The cutting jet will therefore passthrough a liquid between the nozzle outlet and the casing wall. Howeverthis results in a lot of the initial pressure energy of the cutting jetbeing lost through impact loss when the cutting jet collides with theliquid in the casing at high speed. In some cases the liquid filledcasing is therefore arranged with a small pipe volume that is filledwith air or nitrogen, the pipe volume being arranged immediately belowthe cutting tool and comprising the cutting site in question. Said airor nitrogen is hereinafter simply termed a gas. In principle, thecutting jet will thereby pass through gas instead of liquid, wherebysaid impact loss is reduced considerably. By so doing, a significantlygreater share of the initial pressure energy of the cutting jet shouldbe available for cutting the casings and any annular cement. Inprinciple, it should then be possible to cut through pipes and anyannular cement much more quickly, whereby any disruptive or damaginginfluential forces have considerably less time to affect the cuttingresult in a negative manner. Said influential forces may arise as aresult of flow movements or hydrostatic pressure changes in the liquidcolumn above the cutting tool. The influential forces may cause thecutting tool and the cutting jet exiting from it to be subjected toundesirable axial movement, which causes an undesirable reduction incutting power and the precision of the cut. This may cause thecontinuity of the cutting to be interrupted and/or cause the resultingfaces of the cut to form a discontinuous, e.g. helical, cut in stead ofa continuous and circular cut. In both cases the cutting must berepeated. Such movement may also cause fluid leaks in the gaskets of thecutting tool, whereby seeping liquid flows into the cutting area inquestion, possibly reducing the impact force of the cutting jet.

In order to allow said pipe volume to be filled with said gas, thecutting tool must be connected to a compressor on the surface facilityvia a pressure line for gas. According to prior art, the cutting tool isalso equipped with a short drain pipe running through the cutting tool.The upper end of the drain pipe is terminated immediately above thecutting tool, and the lower end of the pipe is terminated below thecutting depth in question. Moreover, the drain pipe is designed to beperipherally rotatable together with the high pressure pipe and the highpressure nozzle, to prevent the cutting jet from cutting off the drainpipe during rotation.

After the cutting tool according to prior art has been fixed in apressure tight manner in the innermost casing of the well, pressurisedgas is pumped into said pipe volume underneath the sealing means of thecutting tool via said pressure line. The gas is supplied at a pressurewhich is sufficient to force water in this pipe volume out through theshort drain pipe in the cutting tool, to be mixed with the surroundingwater immediately above the cutting tool. By so doing, the pipe volumecomprising the cutting depth in question is filled with pressurised gas.During the cutting, pressurised gas is continuously pumped into thispipe volume.

Even though the known technique of hydraulic cutting in a gas filledenvironment is more efficient than cutting in liquid, the knowntechnique of cutting in gas is also encumbered with considerabledisadvantages. Among them is the fact that a continuous feed ofpressurised gas via said small pipe volume will also entail a continuousoutflow of pressurised gas at the top of said short drain pipe. Thus,gas bubbles will continuously rise and expand in the overlying liquidcolumn of the casing. Expansion of gas bubbles in the liquid column maycause percussions or movements in the liquid column, and suchinfluential forces may propagate downwards in the liquid column,possibly causing unwanted movement of the cutting tool during thecutting, cf. previous mention of this. Continuous outflow of gasimmediately above the cutting tool also means that the gas pressure inthe cutting area in question can not exceed the hydrostatic pressure atthe outlet of the short drain pipe to any appreciable extent. Cutting insaid gas filled volume is therefore carried out at a marginal gasoverpressure. In addition, this gas overpressure will remain roughlyunchanged even if the gas inflow rate to the pipe volume is increased.Instead, such an increase will cause a greater outflow of undesirablegas bubbles rising and expanding in the liquid column of the casing. Inaddition to these disadvantages, the marginal gas overpressure is also aconsiderable disadvantage to the hydraulic cutting. When the fluid jetcuts through casings and possibly annular cement, the marginal gasoverpressure will be insufficient to prevent hydrostatically pressuredliquid from the outside of the casing/casings from trickling into thegas filled casing volume via one or more cuts in said casing. Thus thecutting jet will collide with inflowing liquid, causing an impact lossto the cutting jet, which reduces the impact force of the cutting jet.This reduction in the inherent energy of the cutting jet is particularlydisadvantageous when cutting through several consecutive casing sizes,as this loss of energy reduces the ability of the cutting jet to cutefficiently through all the casings and any associated annular cement.

SUMMARY OF THE INVENTION

The object of the present invention is to remedy the above disadvantagesconnected with known hydraulic cutting techniques for cutting of tubularobjects beneath a water floor. Such tubular objects consist of e.g.casings, piles or caissons, such tubular objects hereinafter simplybeing termed pipes. In particular, the invention seeks to remedy thedisadvantages connected with hydraulic cutting in an air or nitrogenfilled pipe volume having a marginal gas overpressure with respect tothe surrounding hydrostatic pressure.

The object is achieved by the characteristics given in the followingdescription and in the appended claims.

The present invention comprises among other things the use of a knownhydraulic cutting system connected to a surface facility, such a cuttingsystem comprising equipment such as mentioned above. The hydrauliccutting system comprises among other things a cutting tool, which in theworking position is anchored in a pressure tight manner in the pipe inquestion, and which in the working position is connected to a compressoron the surface facility. The compressor is used to pump pressurised gas,i.e. either compressed air or compressed nitrogen, in immediately belowthe sealing means of the cutting tool, whereby liquid in this area ofthe pipe is evacuated via a drain line through the cutting tool. Bycontinuing to pump pressurised gas in under the cutting tool, saidliquid will be forced down in the pipe until its surface levels out atthe same level as the inlet to the drain line. By so doing, there willexist a small gas filled pipe volume between said sealing means and theinlet to the drain line, this pipe volume also comprising the cuttingsite in question. Even though said constructional features and steps ofaction are included by prior art, they are prerequisites for theimplementation of the present invention.

According to prior art said drain line through the cutting tool consistsof a short drain pipe, the upper end of which is terminated immediatelyabove the cutting tool, while its lower end is terminated just below thecutting depth in question. As mentioned, this leads to gas bubblesrising through the liquid column of the pipe, and such gas bubbles mayhave a disruptive or damaging effect on the result of the hydrauliccutting. Use of such a short drain pipe also cause the cutting to becarried out at a marginal gas overpressure, allowing the inherent energyof the cutting jet to be reduced through impact losses.

However the present device for a cutting tool is characterized in thatsaid drain line extends further up to the surface facility, where theupper end portion of the drain line is connected to at least oneadjustable fluid choke device, e.g. a choke valve. Liquid and/orpressurised gas will thereby flow up to the surface through the drainline instead of rising through the liquid column of the pipe.Controlling the gas feed rate to said compressor and/or controlling thefluid outflow rate through the choke device(s) of the drain line, willat least allow the pressure of said gas filled pipe volume to becontrolled. By so doing, the pipe volume may be set at a significantlyhigher gas overpressure than said marginal gas overpressure usedaccording to prior art, as this gas overpressure must be seen inrelation to the greatest hydrostatic pressure that exists immediatelyoutside the pipe/pipes. Such hydrostatic pressure may be created by thehydrostatic pressure of the ground formation or by the hydrostaticpressure in the annulus/annuli surrounding the pipe/pipes. When cuttingcompound pipes and possibly annular cement, said gas overpressure mayoptionally be increased further. When a cutting jet passes through suchincreased gas overpressure and cuts through one or more pipes,overpressurised gas will flow out through the cut(s) and force incomingliquid away from the cut(s) in the pipe/pipes, which minimises theliquid seepage towards and through the cut(s).

Liquid that is introduced to said pipe volume from the cutting jet orvia seepage of liquid, will as a result of the elevated gasoverpressure, be drained continuously to the surface facility via saiddrain line. Consequently, fluids carried out by the drain line may,depending on the rate of liquid admission and the gas overpressure inthis pipe volume, consist of liquid, liquid mixed in with pressurisedgas or only pressurised gas. During the cutting however, there must beinteraction between the gas feed rate and the fluid outflow rate. Thisinteraction may be monitored and controlled by means of suitable devicesand equipment associated with the surface facility and/or the cuttingtool. Conveniently, the interaction is arranged by connecting the upperend portion of said drain line to at least one pressure gauge, aknock-out drum designed with at least one fluid choke device, andpossibly also at least one flow meter, and similar equipment forcontrol, treatment and monitoring of the outflowing fluids. As anoption, the cutting tool may also be associated with at least onepressure gauge that measures the gas pressure in said pipe volume duringthe cutting. Moreover, the drain line may be provided with at least oneliquid level indicator that measures the level of said liquid surfacebelow the cutting tool and with respect to a specific point ofreference, e.g. relative to the inlet to the drain line. By so doing,the extent of said pipe volume may be determined continuously during thecutting.

By using a device according to the invention it is avoided that gasbubbles rising through the liquid column of the pipe and causing anydisruptive or damaging movement of the hydraulic cutting tool, whichwould have a negative effect on the result of the hydraulic cutting.

Moreover, use of the present device allows a small pipe volume under thecutting tool to be filled, in a controlled manner, with gas at asignificantly higher pressure than the hydrostatic pressure at thecutting site in question. Consequently, an optimal share of the initialpressure energy of the abrasive fluid will be transmitted to the pipewall in the form of an impact force, so as to provide quick andefficient cutting of the pipe wall and any additional pipes locatedoutside of this. As a result, optimal operating conditions are provided,which increase the likelihood of achieving efficient and successfulhydraulic cuts, and also reduce the operational costs considerablyrelative to known methods of cutting.

The invention also means that hydraulic cutting may be carried out atconsiderably greater water depths than those which are common with knowncutting techniques. In practice, this means that such cutting may becarried out at water depths exceeding 100 metres.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following description and with reference to the appendeddrawings, each reference number will refer to the same detail in alldrawings in which the detail is shown, where:

FIG. 1 is a schematic view of an offshore platform installed on a seafloor, which platform is associated with a well in which hydrauliccutting of the casing of the well is carried out according to previouslyknown techniques; and

FIG. 2 is a schematic view in which hydraulic cutting of the casing ofthe well is carried out by using the present invention in combinationwith the hydraulic cutting technique illustrated in FIG. 1.

Said figures only show those technical details that directly concern theinvention and the understanding of this. In addition, all the drawingsare simplified and distorted with regard to technical details andrelative dimensions.

DETAILED DESCRIPTION

The following examples concerns hydraulic cutting of the casing of awell beneath a water floor in connection with permanent plugging andabandonment of the well.

FIG. 1 and FIG. 2 show an offshore platform 2 installed on the seafloor, which platform is equipped with platform legs 4, and which isarranged over a surface 6 of the sea. The platform legs 4 extend throughseawater 8 down to a sea floor 10 where they penetrate an underlyingground formation 12. An offshore well 14 is formed in the groundformation 12 and extends up to the platform 2. Such a platform 2 willnormally be tied in to more offshore wells 14, but the figures and thefollowing discussion are simplified by referring only to one offshorewell 14.

Before the well 14 is permanently abandoned, all removable equipment isremoved from the well 14, including the wellhead and all or parts of theproduction tubing. After that, the well 14 consists only of casingstrings that are permanently placed in the ground formation 12, andwhich project above the sea floor 10. These are the casing strings thatare cut immediately below the sea floor 10, and where the cut off casingparts are then removed from the sea floor 10. Such casing strings arehereinafter just termed casings.

In the figures, the well 14 consists of several casings placed insideeach other and extending deeper into the ground formation 12 withsuccessively decreasing pipe diameters. In the examples, the pipeassembly consists of a conductor casing 16 (outermost), a surface casing18 and an inner casing 20. The inner casing 20 may for instance be aso-called intermediate casing. In addition, annulus 22 and annulus 24between said casings are filled with set cement 26 that binds the pipestogether, and which forms a pressure barrier against any underlyingreservoir fluids. Moreover, the inner casing 20 is provided with variousdeeper well plugs (not shown in the figures). In the figures, annuli 22,24 are shown as being filled with cement 26 up to just under theplatform 2, while the inner casing 20 is filled with seawater 8 nearlyup to the platform 2. Above the cement 26 and the seawater 8 there isatmospheric air 28.

To begin with, a hydraulic cutting tool 30 that is known per se islowered to a cutting depth 32 in the inner casing 20. The cutting depth32 will normally be approximately 5 metres below the sea floor 10. Thecutting tool 30 is lowered on a cable 34 coupled to a winch 36 on theplatform 2. When lowered into the well 14, the cutting tool 30 is alsoconnected to the platform 2 via a high pressure line 38, a compressedair line 40, two hydraulic lines 42 and 44, and also a monitoring cable46 for electronic monitoring of the hydraulic cutting. The cutting tool30 is shown in the working position in both FIG. 1 and FIG. 2.

According to prior art, the high pressure line 38 is connected to amixing tank 48 and an upstream high pressure pump 50 on the platform 2.Water 52 is pumped from the pump 50 into the mixing tank 48, and in themixing tank 48 the water 52 is mixed with an abrasive 54 to form anabrasive fluid 56. Then the abrasive fluid 56 is pumped down through thehigh pressure line 38, through the cutting tool 30 and out through ahigh pressure nozzle 58 provided for this. The abrasive fluid 56 exitsat a very high speed and forms a cutting jet 60 that cuts through thecasings 16, 18, 20 and said annular cement 26.

In principle, and with reference to FIG. 1, the known cutting tool 30consists of a body 62 with an outer diameter that fits into the innercasing 20; an angular high pressure pipe 64 that projects down from thebody 62 when in the working position, and which is connected by its freeend to said high pressure nozzle 58; as well as a short drain pipe 66extending through the body 62. In the working position the inlet 68 tothe drain pipe 66 is arranged at a deeper position than said cuttingdepth 32, while the outlet 70 of the drain pipe 66 is arrangedimmediately above the cutting tool 30. The body 62 is also equipped withother known equipment that is not shown in the appended drawings. Thisequipment includes among other things a hydraulic rotating motor andrelated equipment used during the cutting to rotate the high pressurepipe 64 and the drain pipe 66 through at least one complete rotationabout the axis of the inner casing 20. Said equipment (not shown) alsocomprises an actuator device for fixing the cutting tool 30 against thepipe wall of the inner casing 20 in a releasable and pressure tightmanner, together with necessary piping, couplings, gaskets and similarconnecting means. The actuator device comprises hydraulic cylinders andpistons that upon activation are forced axially against rubber elasticpacking elements 72 and 74 in the outer wall of the body 62, whereby theelements 72, 74 expand against the inner casing 20 in a pressure tightmanner. Said rotating motor and actuator device (not shown) are drivenby means of hydraulic fluid supplied via said two hydraulic lines 42,44, the lines 42, 44 being connected to at least one hydraulic power andcontrol unit 76 on the platform 2. Also, the body 62 is a unit that isconnected to associated external equipment in a pressure tight manner.In the working position, the cutting tool 30 thereby forms a pressuretight barrier between an overlying section 78 and an underlying section80 of the inner casing 20, and consequently said short drain pipe 66represents the only hydraulic connection between the pipe sections 78,80.

Moreover, the upper end of said compressed air line 40 is connected toan air compressor 82 on the platform 2. The compressed air line 40extends through the body 62 and terminates at a lower outlet 84 locatedimmediately below the body 62. By using the compressor 82, and after thecutting tool 30 has been anchored in the working position in the innercasing 20, pressurised air 86 is continuously pumped out through theoutlet 84 of the compressed air line 40. Seawater 28 in the underlyingpipe section 80 will then be evacuated through the short drain pipe 66,whereby the water 28 will flow out through the outlet 70 of the drainpipe 66 immediately above the cutting tool 30. The liquid outflow willcontinue until its liquid surface 88 in the underlying pipe section 80has been forced down to the inlet 68 to the drain pipe 66. After thatthe outflow will mainly consist of compressed air 86, or of compressedair 86 mixed in with seeping seawater 28 and/or abrasive fluid 56.Therefore, during the cutting operation there will exist an air filledpipe volume 90 between the packing elements 72, 74 and the liquidsurface 88. This drain pipe arrangement will however mean that the airpressure in the pipe volume 90 can not exceed the greatest hydrostaticpressure that exists either at the outlet 70 of said drain pipe 66, insaid annuli 22, 24 or in the surrounding ground formation 12, to anyappreciable extent. As mentioned previously, hydraulic cutting at such amarginal air overpressure will negatively affect the result of thecutting.

In the following, and with reference to FIG. 2, reference will be madeto an embodiment of the present invention. With the exception of saidshort drain pipe 66, the following embodiment comprises among otherthings the same equipment as that mentioned in the preceding and knownembodiment, including said rotating motor, setting device, compressedair means and casing assembly 16, 18, 20. FIG. 2 also shows that cuttingtool 30 in the working position, the cutting jet 60 passing through anair filled pipe volume 90 and cutting through said casings 16, 18 and 20and cement 26.

According to the invention, the cutting tool 30 is also connected to theplatform 2 via a drain hose 92. The lower (upstream) end of the drainhose 92 is connected to the short drain pipe 66 of the body 62, and theupper (downstream) end of the drain hose 92 is connected to a pressuregauge 94 and an adjustable choke device on the platform 2. The chokedevice comprises a knock-out drum 96 to which is connected an air outletpipe 98 and a liquid outlet pipe 100. The air outlet pipe 98 is equippedwith an air choke valve 102, while the liquid outlet pipe 100 isequipped with a liquid choke valve 104 and a liquid flow meter 106.Fluids (liquid 8, 56 and/or compressed air 86) that are drained fromsaid pipe volume 90 via the drain pipe 66 and the drain hose 92 duringthe hydraulic cutting, will be separated into two branch flows in theknock-out drum 96, of which one air branch flow exits through the airoutlet pipe 98 and one liquid branch flow exits through the liquidoutlet pipe 100.

As mentioned, the invention makes it possible to carry out hydrauliccutting at an elevated air overpressure in said pipe volume 90. This airoverpressure may be set at an appropriate pressure level throughinteraction between the air feed rate and the air outflow rate. Theinteraction is implemented through control of the air feed rate from theair compressor 82 and/or by choking the air outflow rate through the airchoke valve 102 in the air outflow pipe 98. The air pressure in the pipevolume 90 is measured by means of said pressure gauge 94.

In addition, the level of the liquid surface 88 in the pipe volume 90may be controlled through interaction between the air pressure in thepipe volume 90 and the liquid outflow rate therefrom. The liquid outflowrate is controlled at the downstream end by means of said liquid chokevalve 104 provided in the liquid outflow pipe 100. This outflow rate ismeasured by means of said liquid flow meter 106.

By monitoring the types of fluid that flow out via the drain hose 92, itis possible to obtain an indication of where in the inner casing section80 the liquid surface 88 is located, in relation to the inlet 68 to saiddrain pipe 66. A discharge consisting only of liquid, e.g. seawater 8and/or abrasive fluid 56, indicates that the liquid surface 88 islocated at a shallower level than said inlet 68. A discharge comprisinga mixture of said liquid and compressed air 86 indicates that the liquidsurface 88 is located at approximately the same level as the inlet 68. Adischarge consisting only of compressed air 86 indicates that the liquidsurface is located at a deeper level than the inlet 68, which conditioncomplicates the measurement of the volume of liquid drained.

Ideally, the liquid surface 88 should be at the same level as the inlet68. With this, the drained liquid volume may be measured at any time,which volume also indicates how much liquid 8, 56 is being introduced tothe pipe volume 90 at any time during the cutting. Based on informationregarding air pressure, outflow rate and type of fluid, it is possibleto e.g. control the air pressure in the pipe volume 90 and/or the levelof the liquid surface 88 in the inner casing section 80. By so doing, itbecomes possible to provide optimal operating conditions during thecutting operation, which increases the likelihood of achieving efficientand successful hydraulic cuts. Said changes are made possible by usingthe present invention.

1. A device for a hydraulic cutting tool for cutting at least one pipebeneath a water floor, the at least one pipe being disposed in a groundformation, wherein hydraulic cutting is carried out from a surfacefacility equipped with at least the following auxiliary equipment: (a) ahoisting device for hoisting the cutting tool down to or up from acutting depth in the pipe; (b) a high pressure pump for pumping anabrasive fluid from an associated mixing tank; (c) a compressor forpumping pressurised gas; and (d) at least one power and control unit forsupplying power to and controlling a releasable setting device and arotating motor in the cutting tool; wherein the cutting tool is made upof: (e) a body fitted with at least the following equipment: (f) saidreleasable setting device for pressure tight setting of the cutting toolin the pipe, whereby the pipe is divided into an overlying pipe sectionand an underlying pipe section; (g) a rotatable high pressure pipe, thefree end of which is connected to a high pressure nozzle from which saidabrasive fluid exits in the form of a cutting jet during the cutting,the high pressure pipe projecting down from the body when in the workingposition; (h) said rotating motor for pipe peripheral rotation of thehigh pressure pipe and the high pressure nozzle during the cutting; and(i) a short drain line extending axially through the cutting tool, theinlet to which, in the working position, is arranged deeper than thehigh pressure nozzle, while its outlet is arranged immediately above thebody, the short drain line thereby forming the only hydraulic connectionbetween said pipe sections of the pipe; wherein said equipment at thecutting tool is connected to said auxiliary equipment on the surfacefacility via the following connecting lines: (j) a hoisting cablebetween the cutting tool and the hoisting device; (k) a high pressureline between the high pressure pipe and the mixing tank; (l) a pressureline for gas leading from said underlying pipe section axially throughthe body and up to the compressor; and (m) at least one auxiliary linefor power supply to, control and/or monitoring of equipment in thecutting tool; and where, after the cutting tool has been set but beforethe cutting is initiated, pressurised gas is pumped continuously throughthe pressure line and into the underlying pipe section, whereby liquidis evacuated through the short drain line and the surface of the liquidis forced down to the inlet to the drain line, so as to create a gasfilled pipe volume comprising said cutting depth between the body andsaid inlet, whereupon the hydraulic cutting is initiated by continuouslypumping abrasive fluid through said high pressure line while rotatingthe high pressure pipe and the high pressure nozzle, wherein the outletof said short drain line is connected to a second drain line extendingup to the surface facility, the upper end portion of which drain line isconnected to at least one adjustable choke device, allowing the gasoverpressure in said pipe volume to be controlled during the cutting inorder to achieve optimal cutting conditions.
 2. A device in accordancewith claim 1, wherein a upper end portion of the second drain line isconnected to at least one pressure gauge.
 3. A device in accordance withclaim 1, wherein the cutting tool is associated with at least onepressure gauge measuring the gas pressure in said pipe volume.
 4. Adevice in accordance with claim 2, wherein the cutting tool isassociated with at least one pressure gauge measuring the gas pressurein said pipe volume.
 5. A device in accordance with claim 1, wherein thecutting tool is associated with a liquid level indicator that measuresthe level of the liquid surface below the cutting tool, whereby theextent of said pipe volume may be determined continuously during thecutting.
 6. A device in accordance with claim 2, wherein the cuttingtool is associated with a liquid level indicator that measures the levelof the liquid surface below the cutting tool, whereby the extent of saidpipe volume may be determined continuously during the cutting.
 7. Adevice in accordance with claim 3, wherein the cutting tool isassociated with a liquid level indicator that measures the level of theliquid surface below the cutting tool, whereby the extent of said pipevolume may be determined continuously during the cutting.
 8. A device inaccordance with claim 1, wherein said at least one adjustable chokedevice is constituted by a knock-out drum to which the drain line isconnected, the knock-out drum being connected at its downstream side toa separate gas outlet pipe with a gas choke valve and a separate liquidoutlet pipe with a liquid choke valve.
 9. A device in accordance withclaim 8, wherein the liquid outlet pipe is equipped with at least oneliquid flow meter.